RUBINSTEIN, J. 2021. OTHERWORLDLY WINGNUTS: PTEROCARYA x REHDERIANA. ARNOLDIA, 78(5-6): 68-69 
Otherworldly Wingnuts: Pterocarya x rehderiana 
Jared Rubinstein 
rarely visited corner at the Arnold Arbo- 
retum is nestled beneath the tall stone 
wall that separates the hickory collec- 
tion from traffic on Centre Street. In late sum- 
mer, the area feels otherworldly. The heavy 
overstory filters the light and cools the air; the 
humidity seems to increase; and densely planted 
shrubs block out the surrounding views and 
noises. The corner is dominated by a planting 
of seemingly colossal hybrid wingnuts (Ptero- 
carya x rehderiana), with their drooping Span- 
ish moss-like fruits and twisted forms. Standing 
next to their large multistemmed trunks can 
make you feel miniature. 
Wingnuts are closely related to hickories 
(Carya) and walnuts (Juglans). There are six spe- 
cies of Pterocarya, with native ranges clustered 
in China, Japan, Southeast Asia, and the Cau- 
cuses. In addition to cultivating representatives 
of five of the six species, the Arnold Arboretum 
has eight specimens of this unusual hybrid, all 
of which grow in this out-of-the-way corner. 
The oldest of the eight originated at the 
Arboretum from seed sent, in 1879, by Pierre 
Alphonse Lavallée of the Arboretum de Segrez, 
outside of Paris. At the time, the Arboretum de 
Segrez was one of the largest in the world (and 
a noteworthy landscape where Marcel Proust 
once suffered an asthma attack but still man- 
aged to write a poem about its beauty). Lavallée 
collected the seeds from a Chinese wingnut 
(P. stenoptera) in his arboretum, and, once 
they germinated in Boston, the seedlings were 
planted along Centre Street. 
Two decades later, Alfred Rehder, an Arnold 
Arboretum taxonomist, noticed that the trees 
didn’t look quite like the Chinese wingnut. 
“The trees in the Arnold, known as Pterocarya 
stenoptera ... | can no longer consider, after 
much study, as the real species of that name,” 
Rehder wrote to the German Dendrological 
Society in 1903, “but now consider [them] a 
cross between this and P. fraxinifolia |the Cau- 
casian wingnut], which in its characteristics 
almost exactly stops between the two species.” 
Rehder hypothesized that pollen from a Cau- 
casian wingnut growing at the Arboretum de 
Segrez must have landed on the flowers of a 
Chinese wingnut growing nearby. We don’t 
know who collected and brought the Chinese 
and Caucasian wingnuts to Paris, but it may 
well have been the first time that the two spe- 
cies, normally separated by the thousands of 
miles between the Caucasus Mountains and 
eastern China, were growing in the same place. 
Rehder conferred with Camillo Schneider, a 
taxonomist working at the Vienna Natural His- 
tory Museum, who agreed with Rehder’s assess- 
ment. Based on their correspondence, Schneider 
published the first botanical description of the 
new hybrid in 1906. Writing in German in the 
Ilustriertes Handbuch der Laubholzkunde, 
he identified the unique characteristics of the 
buds and rachises of the “Bastardes” growing at 
the Arnold Arboretum and officially named the 
hybrid for his friend, choosing the Latin name 
Pterocarya x rehderiana. 
Four trees (accession 1191) from Lavallée’s 
1879 shipment still grow along the Centre 
Street wall, hidden behind the hickory collec- 
tion. In addition, four neighboring trees (23119) 
were accessioned as seedlings from the original 
trees. When the wingnuts fruit in midsummer, 
they offer a dazzling display of long, pendulous 
clusters of winged nutlets (hence the com- 
mon name) that dangle from what seems like 
every branch. One particularly large specimen, 
accession 1191*E, has an incredible form, with 
leaders that shoot up more than 125 feet and 
droop over the Works Progress Administration- 
constructed bus shelter on Centre Street. 
As with many hybrids, Pterocarya x rehderi- 
ana seems to display hybrid vigor and, according 
to Rehder, are “much hardier and more satisfac- 
tory than their supposed parents.” A windstorm 
in October 2020 took out one of the leaders 
from accession 1191*E, but overall, the hybrids 
don’t seem terribly affected by the cold New 
England winter, even after more than 140 years 
growing at the Arboretum. While the hybrids 
are a product of a chance cross that would likely 
have never been possible in the wild, the trees 
have more than claimed their uncanny home. 
Jared Rubinstein is an associate project manager at the 
Arnold Arboretum. For more on the taxonomic history 
of the Rehder wingnut, see his 2020 article with Michael 
Dosmann in Novon, issue 28(4). 
